Vibrations can cause unwanted noise, material fatigue, and even catastrophic failure of systems. In response, active vibration control systems are sometimes employed. In general, the control system detects the amplitude and frequency of the vibrations emanating from a vibrating source and attempts to cancel them out by imparting canceling forces.
In one prior art system, vibrations of the rotor, gear box, and engine assembly of a helicopter (e.g., the Westland-Agusta EH-101 Merlin helicopter) are controlled by a system including accelerometers on the helicopter fuselage which detect vibrations; actuators between the fuselage and the rotor, gear box, and engine assembly; and a processing system which drives the actuators in response to the accelerometer signals. See, for example, U.S. Pat. No. 5,853,144 incorporated herein by this reference.
There are two primary problems associated with this and other prior art active vibration control systems. The first is complexity. Typically, numerous accelerometers are required on different portions of the fuselage and each accelerometer has a different output signal since it detects vibrations of different amplitude and frequency depending on its location on the fuselage. Processing all the accelerometer signals and then deciding how to properly drive the actuators in response is very difficult and results in severe computational complexity.
Second, such a system is based on the premise that the vibrations are periodic comprising one or more steady sinusoidal behaviors that are summed together. In truth, they are not periodic: during each revolution of the rotor, the amplitude of the vibrations change, and as between successive rotations, the vibrations are not the same even when the rotor speed and airspeed remain constant. The result is that present systems do not always sufficiently cancel the resulting vibrations when employed with sources of vibrations exhibiting irregular chaotic behavior.